The invention relates to an X-ray apparatus which includes an X-ray source for producing X-rays, an X-ray detector for detecting the X-rays, a filter which is arranged between the X-ray source and the X-ray detector and includes a plurality of tubular filter elements having a longitudinal direction z and a circumference, the X-ray apparatus also including an electrical device which is provided with at least one power supply source for controlling the individual filter elements, each filter element having an internal volume for receiving a liquid filling having electrically conductive and X-ray absorbing properties, said filter element having an X-ray absorptivity which is dependent on the quantity of X-ray absorbing liquid present in the internal volume, each filter element being provided with a first electrode for applying a first electric potential to a wall of the filter element and a second electrode for applying a second electric potential to the internal volume of the filter element, and the X-ray absorptivity of each filter element being adjustable by step-wise control of a level of the X-ray absorbing liquid in the longitudinal direction z of the filter element.
An X-ray apparatus of this kind which includes an X-ray filter is known from United States patent U.S. Pat. No. 5,666,396 (PHN 15.378). The known X-ray apparatus comprises a filter with a plurality of filter elements having individual absorptivities which are dependent on a level of a liquid filling present in the filter element. The X-ray apparatus is used inter alia for medical diagnosis where a patient to be examined is arranged between the X-ray source and the X-ray detector so as to image internal structures. Thanks to the fact that the patient has structures of different electron density, regions of different density are observed in a resultant X-ray image. The interval in density between regions with the extremes of the density in an X-ray image is defined as the dynamic range. The filter serves to limit a dynamic range for each X-ray image.
In order to limit the dynamic range of the object to be examined, the known X-ray apparatus includes a filter with filter elements provided with a bundle of tubes for receiving a liquid filling which is X-ray absorbing as well as electrically conductive, each tube being connected to a common supply channel. Each filter element is provided with a first electrode which is arranged in a wall of the filter element in order to apply an electric voltage to the wall of the filter element. A second electrode is in contact with the liquid filling. The electric voltage applied to the first electrode of the filter element influences the adhesion between the liquid filling and an inner wall of the filter element; this adhesion determines whether the relevant filter element is filled with the liquid filling. The relative quantity of liquid filling in individual filter elements is controlled on the basis of the electric voltages applied to individual filter elements. The known apparatus utilizes the phenomenon that a contact angle between an electrically conductive liquid and an electrode which is insulated therefrom is changed by applying a potential difference between the electrically conductive liquid and the electrode. This phenomenon is known as electrowetting. When electrowetting is applied to a tubular filter element which has an electrode arranged in the wall and is filled with an electrically conductive liquid filling, the level of the liquid filling in the filter element can be influenced thanks to the fact that the electrowetting force is directed in the longitudinal direction of the filter element so that the degree of filling of the filter element can be increased or decreased as desired. In order to enable the potential difference to be realized between the liquid filling and the first electrode in the wall of the filter element, they are electrically insulated from one another by means of an insulation layer provided on the inner wall of the filter element. The known apparatus operates as follows: in the presence of a first value of the electric voltage, the adhesion of the liquid filling to the inner wall is increased and the relevant filter element is filled with the liquid filling from the supply channel. In the presence of a second value of the electric voltage the adhesion is reduced and the liquid filling is discharged from the filter element to the supply channel. Filter elements are adjusted to a high X-ray absorptivity by filling the elements with the liquid filling; filter elements are adjusted to a low X-ray absorptivity by keeping the filter elements empty.
It is a drawback of the known device that the filling of each filter element is controlled by application of a sequence of electric voltage pulses to the first electrode of the filter element so that the filter element is electrically charged. The level of the electric charge defines the degree of filling of the filter element. It has been found that the filling level of the filter element becomes poorly reproducible in the course of time. In practice it is often desirable to make the filling reproducible with an a priori known degree of discretization in order to achieve a reliable range of grey values.
It is an object of the invention to provide an X-ray apparatus which includes a filter provided with filter elements whose X-ray absorptivity can be step-wise controlled in a reliable and reproducible manner. To this end, an X-ray apparatus according to the invention is characterized in that the first electrode includes a series of electrode segments which extend at least over a part of the circumference of the filter element and succeed one another in the longitudinal direction of the filter element, directly successive electrode segments being electrically insulated from one another and individually controllable by the electrical device. This step offers the desired effect in that each filter segment includes a stack of electrode segments in the longitudinal direction, the degree of segmentation being known a priori. Thanks to the fact that the electrode segments are electrically insulated from one another, the filter element can be step-wise filled with the liquid filling. The absolute value of the electric voltage applied to an individual electrode segment can be chosen to be such that the secondary effects on the reproducibility of the filling, that is, the orientation of the filter element relative to the force of gravity and the aging effects of the cover layer (a protective layer between the insulator layer and the liquid filling) are compensated. A number of different possibilities can be distinguished for explaining the process of the step-wise filling of the filter element. In order to realize a liquid filling having electrically conductive and X-ray absorbing properties, an electrically conductive and X-ray absorbing salt can be dissolved in a liquid (for example, water). This solution, consisting of one component, thus has electrically conductive as well as X-ray absorbing properties. It is also feasible to realize a liquid filling having said properties by dissolving a first substance having exclusively electrically conductive properties and a second substance having exclusively X-ray absorbing properties. The resultant liquid filling thus contains fully miscible liquid components. Another possibility is to form said liquid filling from a number of, for example two non-miscible liquid components, one of which has the required electrically conductive properties while the other has the required X-ray absorbing properties.
The operation of the filter element will now be described first of all for the first two above-mentioned cases where the liquid filling consists of one liquid which is electrically conductive as well as X-ray absorbing, for example a salt solution or a liquid metal. It is also assumed that the filter element is initially empty and that no potential difference is present as yet between the liquid filling and the first electrode. Furthermore, a distinction is made between a xe2x80x9cfillingxe2x80x9d voltage for completely filling the electrode segment and a xe2x80x9cdrainingxe2x80x9d voltage for draining the electrode segment. In order to fill the filter element up to an nth electrode segment, a xe2x80x9cfillingxe2x80x9d voltage must be applied to all electrode segments up to and including nxe2x88x921. In order to make the liquid level rise further, the xe2x80x9cfillingxe2x80x9d voltage must be applied to the directly subsequent electrode segment. For optimum effect of this control chart it is advantageous when the directly successive segments are arranged in the vicinity of one another in such a manner that the meniscus of the liquid filling in the preceding electrode segment experiences an electric field of the directly subsequent segment, provided that an electric potential has been applied to the directly subsequent electrode segment. A second advantageous step for achieving an optimum effect of the filter element consists in creating a given time overlap during which the xe2x80x9cfillingxe2x80x9d voltage is applied to both directly successive electrode segments. As soon as the meniscus of the liquid filling is present in the next electrode segment, the electric potential of the preceding electrode segment may be adjusted to the value of the xe2x80x9cdrainingxe2x80x9d voltage.
As has already been stated, the liquid filling may also be composed of more, notably two, non-miscible liquid components. In that case the properties of the liquid components can be individually optimized, so that one liquid component has optimum electrically conductive properties and is hardly X-ray absorbing while the second liquid component has optimum X-ray absorbing properties and is electrically insulating. For an optimum effect of the filter element provided with the liquid filling consisting of two liquid components, the respective liquid columns should adjoin one another so that a common interface is formed in the transverse direction. It is also feasible for the two liquid components to remain separated from one another by a gas layer. Furthermore, it must be possible to supply the liquid components from a respective supply channel. In that case the filter element is always filled with the liquid filling, the degree of X-ray absorption then being determined by the level of the X-ray absorbing liquid component in the filter element. In this case the operation of the filter element is similar to that of the described control chart. According to this method the level of the X-ray absorbing liquid component is determined passively by the level of the electrically conductive liquid component in the filter element and the maximum X-ray absorption is reached when the filter element is completely filled with the X-ray absorbing liquid component.
For ease of control it is also possible to introduce sub-groups of electrode segments. To this end, a first embodiment of the X-ray apparatus according to the invention is characterized in that the electrode segments bearing even sequence numbers in the series and the electrode segments bearing odd sequence numbers in the series constitute respective sub-groups, the electrical device being provided with switches for controlling said sub-groups. In order to achieve the step-wise filling in this configuration it is advantageous to create a time overlap between the xe2x80x9cfillingxe2x80x9d voltage pulses on the directly successive electrode segments and to adjust the value of the electrical voltage to the xe2x80x9cdrainingxe2x80x9d voltage as soon as the meniscus of the liquid filling is present in the next electrode segment.
A further embodiment is characterized in that each electrode segment is connected to the electrical device via a respective connection. This embodiment offers the advantage that the individual electrode segments do not constitute electrical groups, with the result that it is not necessary to adjust the electric voltage of the preceding electrode segment to the value of the xe2x80x9cdrainingxe2x80x9d voltage as soon as the next electrode segment is filled.
It may be advantageous to make the edges of the directly successive segments overlap one another in space. To this end, a third embodiment according to the invention is characterized in that facing edges of directly successive electrode segments are provided with meshing teeth. This step ensures that the meniscus of the absorption liquid is in contact with a part of the edge surface of the next segment, so that the distance between the neighboring segments becomes less critical.